1. Field of the Invention
The present invention relates to a phase delay element for a liquid crystal display, and more particularly, to phase delay element for a transmissive and reflective type liquid crystal display in which the display operation is carried out in a reflection mode of a low power consumption at a bright place where a light amount is abundant and the display operation is also carried out in a transmission mode of a high luminance at a dark place where a light amount is deficient
2. Description of the Related Art
In an information-oriented society these days, the role of an electronic display is getting more important. All kinds of electronic displays are widely used in various industrial fields.
Generally, the electronic display is an apparatus for visually providing a variety of information to a person. In other words, an electrical information signal output from various electronic devices is converted into a visually recognizable optical information signal at the electronic display. Therefore, the electronic display serves as a bridge for connecting the person and the electronic devices.
Electronic displays are classified as either an emissive display in which the optical information signal is displayed by a light-emitting way, or a non-emissive display in which the optical information signal is displayed by an optical modulation way such as light-reflecting, dispersing and interfering phenomenon, etc. As the emissive display is known as an active display, for example, they include a CRT (Cathode Ray Tube), a PDP (Plasma Display Panel), an LED (Light Emitting Diode) and an ELD (Eelectroluminescent Display), etc. As the non-emissive display is known as a passive display, they include an LCD (Liquid Crystal Display), an ECD (Electrochemical Display) and an EPID (Electrophoretic Image Display), etc.
The CRT used in an image display, such as a television receiver and a monitor, for example, has the highest market share in an aspect of displaying quality and economical efficiency, but also has many disadvantages such as heavy weight, large volume and high power consumption.
Meanwhile, due to rapid developments in semiconductor technology, various kinds of electronic devices are driven by lower voltage and lower power, and thus the electronic equipments became much slimmer and lighter. Therefore, a flat panel type display having the slimmer and lighter characteristic, as well as the lower driving voltage and lower power consumption characteristic, is required according to the new environment.
The LCD among the various developed flat panel type displays is much slimmer and lighter than any other displays, and has a lower driving voltage and lower power consumption, and also has a display quality similar to that of the CRT. Therefore, the LCD is widely used in various electronic equipments.
The LCD is classified as either a transmission type LCD for displaying an image using an external light source such as a backlight assembly, a reflection type LCD for displaying an image using natural light, and a transmissive and reflective type LCD in which the display operates in a transmission mode using an internal light source provided in the display itself when indoors or in a dark place where an external light source does not exist and the display operates in a reflection mode to display an image by reflecting an external incident light in a high brightness environment, such as outdoors.
The reflective type LCD apparatus, in general, displays an image using an external natural light or ambient light that is provided to the LCD apparatus. Therefore, the reflective type LCD apparatus may not display the image when the LCD apparatus is surrounded in darkness.
The transmissive type LCD apparatus displays an image using an artificial light that is generated from a backlight assembly internal to the LCD apparatus. Therefore, the transmissive type LCD apparatus can display the image when the LCD apparatus is surrounded in darkness. However, the transmissive type LCD apparatus has a larger power consumption than the reflective type LCD apparatus. In addition, the transmissive type LCD apparatus has a battery resulting in a heavier weight than the reflective type LCD apparatus. Therefore, the transmissive type LCD apparatus is not as desirable for use as a portable display apparatus compared with the reflective type LCD apparatus.
The LCD controls the alignment of liquid crystal molecules using a voltage applied to the liquid crystal layer, and can be classified as either a passive matrix type or an active matrix type, depending on the way the pixels are driven. In the passive matrix type, pixels are driven using a root-mean-square (rms) of a difference between voltages applied to signal lines and scanning lines while a line addressing in which a signal voltage is applied to all of the pixels at the same time is carried out. In the active matrix type, pixels are driven by a switching element such as a metal-insulator-metal (MIN) device or a thin film transistor (TFT).
FIG. 1 is a cross-sectional view showing a conventional reflective-transmissive type LCD apparatus. A portion of an artificial light, i.e., from a backlight assembly disposed at a rear side of the reflective-transmissive LCD apparatus, is lost.
Referring to FIG. 1, the reflective-transmissive LCD apparatus includes a lamp 1, a lamp reflecting plate 2, a lower polarizer 3, a retardation film 4, a reflection layer 5, a liquid crystal layer 6, a color filter 7, and an upper polarizer 8.
The lamp 1 is disposed on a backside of the lower polarizer 3 and intermediate thereof and the lamp reflecting plate 2. Lamp 1 supplies the lower polarizer 3 with an artificial light. The lower polarizer 3 has an absorption axis that is substantially perpendicular to a horizontal direction defining substantially parallel layers with respect to the reflective-transmissive LCD apparatus. When the artificial light generated from the lamp 1 is incident on the lower polarizer 3, a portion of the artificial light vibrating in the horizontal direction passes through the lower polarizer 3 and is emitted towards a viewer's side of the reflective-transmissive LCD apparatus. When the natural light that is provided from the exterior of the LCD apparatus is incident on the lower polarizer 3, a portion of the natural light vibrating in the horizontal direction passes through the lower polarizer 3 and is emitted towards the backside of the reflective-transmissive LCD apparatus.
The retardation film 4 includes a 1/4 wavelength phase (λ/4) retardation film 4. When the artificial light or the natural light passes through the λ/4 retardation film 4, a phase of the light is delayed by about 1/4 of the wavelength phase or λ/4. The 1/4 wavelength phase retardation film 4 functions to convert a linearly polarized light to a circularly polarized light, or vice versa by causing a phase difference of 1/4 wavelength between two polarization components that are normal to each other and are parallel to optical axes of the 1/4 wavelength phase retardation film 4.
The reflection layer 5 is disposed under the liquid crystal layer 6 and is intermediate the liquid crystal layer and the 1/4 wavelength phase retardation film 4 as illustrated. When a vertically polarized light is incident on the reflection layer 5, the vertically polarized light is reflected from the reflection layer 5. A luminance of the vertically polarized light is controlled by the liquid crystal layer 6. More specifically, the arrangement of the liquid crystal layer 6 varies in response to an electric field applied thereto, thus allowing a light transmittance of the liquid crystal layer 6 to be changed. A portion of the vertically polarized light that passes through the liquid crystal layer is incident on the color filter 7 and, passes through the color filter 7, dependent on a predetermined wavelength range.
The upper polarizer 8 includes a vertical polarizing axis allowing a vertically polarized light to pass through the upper polarizer 8. When the vertically polarized light that is provided from the backside is incident on the upper polarizer 8, the vertically polarized light passes through the upper polarizer 8. In addition, when the natural light or a frontal light is incident on the upper polarizer 8, the vertically polarized light passes through the upper polarizer 8 and is incident on the color filter 7.
The artificial light corresponding to the transmissive mode has a lower efficiency than an efficiency of the natural light corresponding to the reflective mode. When the reflective-transmissive LCD apparatus is in the transmissive mode, the artificial light generated from the lamp 1 is incident on the lower polarizer 3 allowing the linearly polarized light to pass through the lower polarizer 3. The linearly polarized light is incident on the retardation film 4 allowing the right circularly polarized light to be emitted from the retardation film 4. A portion of the right circularly polarized light passes through a transmission window of the liquid crystal layer 6 having a wavelength phase of the light that is changed in response to the electric field applied to the liquid crystal layer 6.
When the right circularly polarized light passes through the liquid crystal layer 6, either the right circularly polarized light or the vertically polarized light is emitted from the liquid crystal layer 6 dependent on the electric field applied to the liquid crystal layer 6. In addition, it is noted that the vertically polarized light passes through the upper polarizer 8, while the right circularly polarized light may not pass through the upper polarizer 8.
A remaining portion of the right circularly polarized light that is emitted from the retardation film 4 is reflected from the reflection layer 5 and emitted therefrom as a left circularly polarized light. The left circularly polarized light is incident on the retardation film 4 so that the vertically polarized light is emitted from the retardation film 4 toward the lower polarizer 3. The vertically polarized light is blocked by the lower polarizer 3. Therefore, the remaining portion of the artificial light is lost, thus decreasing the efficiency of the lamp.
For example, when an effective display area is about 80% and the transmission window is about 30% of the unit pixel, more than about 70% of the unit pixel is therefore lost for transmission of artificial light.
Accordingly, there is a desire to improve a luminance of a reflective-transmissive LCD apparatus by increasing the efficiency of the artificial light reflected from the reflection layer.